In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. User equipment units (UEs) may be, for example, mobile telephones (“cellular” telephones), desktop computers, laptop computers, tablet computers, and/or any other devices with wireless communication capability to communicate voice and/or data with a radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or (in Long Term Evolution) an eNodeB. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations.
In some versions (particularly earlier versions) of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks, typically through a gateway.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. The Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3 GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Specifications for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3 GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller node are performed by the radio base stations nodes. As such, the radio access network of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller nodes.
The evolved UTRAN comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing user-plane and control-plane protocol terminations toward the UEs. The eNB hosts the following functions (among other functions not listed): (1) functions for radio resource management (e.g., radio bearer control, radio admission control), connection mobility control, dynamic resource allocation (scheduling); (2) mobility management entity (MME) including, e.g., distribution of paging message to the eNBs; and (3) User Plane Entity (UPE), including IP Header Compression and encryption of user data streams; termination of U-plane packets for paging reasons, and switching of U-plane for support of UE mobility. The eNB hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The eNodeB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.
The LTE standard is based on multi-carrier based radio access schemes such as Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and SC-FDMA in the uplink. Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which reduces interference. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
As noted above, in the E-UTRAN Radio Access Network scheme, the management of radio resource such as time, frequency and spatial resources takes place in the individual base stations (or cells). Each eNodeB base station therefore includes a Radio Resource Management (RRM) unit for performing management of radio resources. These RRM units typically operate independently from each other, except for very limited exchange of information, such as traffic load condition.
FIG. 1 schematically illustrates a conventional wireless network 10. Referring to FIG. 1, in a conventional wireless network 10, a base station 12 communicates with a core network 18 through a gateway 16. Communications between the base station 12 and the gateway 16 are carried over a transport network 20, which may include wired and/or wireless communication links. The base station 12 also communicates with one or more user equipment units (UEs) 14 through a radio access network (RAN 30). Signals, such as voice and/or data signals, transmitted by the UE 14 are carried over the RAN 30 to the base station 12, and then over the transport network 20 to the gateway 16, for transmission to the core network 18.
Transmissions from the UEs 14 to the base station 12 are referred to as “uplink” transmissions, while transmissions from the base station 12 to the UEs 14 are referred to as “downlink” transmissions. In downlink transmissions, data for multiple UEs are aggregated by the base station 12 into a transmission time interval (TTI), which translates into twelve or fourteen OFDM symbols for more efficient bandwidth usage. The hardware and software in the base station 12 that handle the actual physical transmission of data over the air interface is referred to as the physical (PHY), or L1 layer, of the base station. The L1 layer transmits transport blocks provided by the transport, or L2 layer.